This section provides background information related to the present disclosure which is not necessarily prior art.
Flyback converters are commonly used in low-power applications because they can operate at a wide input voltage range, can provide isolation, require less components, etc. Among flyback converters, variable frequency flyback converters, which are commonly known as quasi-resonant (QR) flyback converters, operate at lower frequencies during full load conditions and increase their operating frequency as the load decreases.
During switch turn-on in a flyback converter, power may be dissipated in the switch through turn-on loss and conduction loss. Quasi-resonant flyback converters can eliminate or partially eliminate turn-on losses by using a valley-switching mode of operation. In quasi-resonant operation, energy stored in a primary MOSFET drain capacitor (i.e., an equivalent capacitance), resonates through a magnetizing inductance of a transformer. A controller can detect a lowest valley of the resonating voltage and turn on the primary MOSFET during the lowest valley to reduce turn-on losses.
During quasi-resonant operation, turn-on loss of the primary MOSFET may be fully eliminated (during full zero-voltage switching (ZVS), or partially eliminated (during partial ZVS)). Full ZVS may or may not be achieved based on the input voltage and a reflected output voltage. For example, ZVS operation is not achieved when the reflected output voltage is lower than a DC input voltage.